The filamentous cyanobacterium Anabaena differentiates specialized cells called heterocysts at regular intervals along its filaments under conditions of aerobic nitrogen limitation. The heterocyst is an anaerobic factory for nitrogen fixation. During differentiation of a vegetative cell into a heterocyst, genes encoding proteins involved in carbon assimilation are turned off while genes encoding nitrogenase proteins are turned on. We have cloned, physically mapped, and sequenced genes of both classes. The organization of the nif genes found in DNA from vegetative cells of Anabaena differs from the organization found in Klebsiella. However, we have recently found that the nif gene- containing DNA of vegetative cells is rearranged during heterocyst differentiation. One event characterized in some detail is the excision of an 11 Kb element that interrupts the nifD gene in vegetative cells, resulting in the fusion of the nif H, D and K genes into a single operon in heterocysts. A second excision event removes a 55 Kb element that is located on the 5' side of the nifS gene in vegetative cells. This results in the fusion of the nifB gene, from a distant location, to the nifS gene in heterocysts. We propose to purify the enzyme(s) responsible for the nifD recombination, to identify the targets for the recombination enzymes and their reaction mechanism, and to study the regulation of the rearrangement systems during Anabaena heterocyst differentiation. The latter studies will include gene fusions to the xisA gene to determine its transcription control, the preparation and use of antibody to the xisA gene product to follow its fate during heterocyst differentiation, and site-specific inactivation of the xisA gene in Anabaena.